The present invention relates to thin film electroluminescent phosphor material.
Thin films of rare earth doped alkaline earth sulfides such as cerium doped strontium sulfide have been extensively investigated for applications in full color alternating current thin film electroluminescent (ACTFEL) display devices. Such a device is disclosed by Barrow et al., U.S. Pat. No. 4,751,427, incorporated by reference herein. The emission spectrum of SrS:Ce is very broad covering both blue and green portions of the visible spectrum, i.e., 440 to 660 nm with a peak at around 500 nm. A full color ACTFEL display device can be obtained by adding a red emitting phosphor, for example CaS:Eu or one that has a red component in its emission spectrum. With such a combination of films, one can build a white light emitting phosphor stack. White phosphor structures can then be laminated with primary color filters to build a color display which is very cost effective in terms of production.
With white light emitting phosphor stacks, however, the blue portion of the emission spectrum can be rather weak, particularly strontium sulfide phosphor doped with cerium which in the past has been one of the most promising of the blue emitting phosphors. Only about 10% of the original luminance can be obtained after filtering if a nearly blue color is to be achieved. For blue coloration in the CIE range of x=0.10, y=0.13 the transmission ratio is further reduced to only about 4%. Therefore, to produced a color display with acceptable luminance, it is necessary to use a lighter blue color filter but this in turn leads to a compromised blue chromaticity. Any display fabricated with such a poor blue chromaticity has a limited color gamut and is unable to produce the range of colors available with CRT or LCD technology.
Therefore, in order to achieve a high performance color ACTFEL display, the blue emission efficiency of the electroluminescent phosphor thin film must be greatly improved. In U.S. Pat. No. 4,725,344, Yocom et al., a method is disclosed for forming alkaline earth sulfide luminescent films by chemical reaction between alkaline earth metal halide and hydrogen sulfide on heated substrates. Yocom et al. does show a strontium sulfide thin film phosphor which has a more bluish color (CIE x=0.17, y=0.25) than an unfiltered SrS:Ce device. However, the luminance performance of the Yocom et al. device is not high enough for practical application. Experimentation has also been reported regarding SrS:Cu devices which are prepared by sputtering, for example in Ohnishi et al., proceedings of the SID 31/1, 31 (1992). The Ohnishi et al. device, however, is even dimmer than the Yocum et al. device (and no color data is available).
Higton et al., U.S. Pat. No. 4,365,184, disclose what is generally known in the art as a powder electroluminescent device. The construction of a powder electroluminescent includes a pair of electrodes with a phosphor layer interposed therebetween. The phosphor layer is a thick film, generally having a thickness of 25 microns or more, which is normally applied in a manner similar to paste. Powder electroluminescent devices are illuminated using a direct current. The use of a direct current between the electrodes is necessary because the powder phosphor layer, as taught by Higton et al., is a semi-insulative material and a large net direct current flow is required for illumination. The core of each phosphor particle is coated, or otherwise formed, with a resistive layer injects carriers into the powder and a much lower average electric field strength than tunneling fields required for the operation of thin film alternating current electroluminescent devices, as previously described. This resistive current then excites the activator atoms in the powder phosphor to emit light. Unfortunately, the characteristics of the resistive layer changes during extended usage which raises its threshold voltage. The increase in the threshold voltage thereby decreases the brightness of the display. If the resistive layer surrounding the particles were removed then the phosphor layer would act as a short circuit rendering the device ineffective. In contrast to ACTFEL devices, the use of an AC signal on a direct current powder device, as taught by Higton et al., would not impose a sufficient voltage on the particles for illumination. Further, if an AC voltage was applied to the powder electroluminescent device disclosed by Higton et al. the efficiency of the device would be extremely low because of the resistance layer. Because of the different operating principles between powder devices and ACTFEL devices, together with different phosphor material characteristics (resistive layer and thickness), one would not consider powder phosphors suitable for thick-film powder devices disclosed by Higton et al. suitable for an ACTFEL device.
Lehmann, in a paper titled "Alkaline Earth Sulfide Phosphorous Activated by Copper, Sulfur, and Gold," reported that strontium sulfides doped with monovalent ions with a d.sup.10 configuration, e.g., Cu.sup.+ and Ag.sup.+, emit green and blue light, respectively, when excited by an electron bombardment. Lehmann was attempting to develop a powder cathode phosphor material suitable for cathodo ray tube devices. Such phosphor powder materials are considered unsuitable for alternating current film electroluminescent devices, such as the device disclosed by Barrow et al.
Vecht et al., in a paper entitled "DC Electroluminescence in Alkaline Earth Sulfides" disclose a powder direct current electroluminescent device using a SrS:Cu powder where the emission is a green color. Like Higton et al., such a phosphor is not suitable for alternating current thin film electroluminescent devices.
Sun et al., U.S. Pat. No. 5,309,070, disclose a (Sr,Ca)Ga.sub.2 S.sub.4 :Ce phosphor for an ACTFEL device. Such a phosphor offers a saturated blue color, e.g., (CIE x=0.15, y=0.10-0.20), but the luminous efficiency is poor, e.g., e40=0.02-0.03 lm/W. In addition, it is extremely difficult to fabricate such a composition as a thin film with good crystallinity at reasonable low substrate temperatures due to its complex chemistry.
A blue emitting SrS:Cu electroluminescent phosphor for alternating current thin-film electroluminescent devices was reported by Kane et al. in a paper entitled "New Electroluminescent Phosphorous Based on Strontium Sulfide." However, the device taught by Kane et al. has a poor performance, e.g., less than 1.0 cd/m.sup.2 at 60 Hertz.
Sun et al., in a paper entitled "A Bright and Efficient New Blue TFEL Phosphor," developed a phosphor, namely, SrS:Cu, with an increased luminous performance over the prior known blue phosphors.
Velthaus et al., in a paper entitled "New Deposition Process for Very Blue and Bright SrS:Ce,Cl TFEL Devices," disclose the use of silver as a co-doping 25 material for SrS:Ce,Cl thin-film electroluminescent devices. Velthaus et al. suggest that the silver co-doping improves the emission spectrum of SrS:Ce electroluminescent devices to a more bluish color. The improvement was attributed to the effect of the Ag.sup.+ charge compensation for the Ce.sup.3+ to eliminate the Sr.sup.2+ vacancies. The mechanism can be thought of as the cerium 3+ replacing the strontium 2+ cites leaving an extra positive charge left over. The strontium vacancies, which are defects, degrade the crystallinity and cause a red shift of the emission which results in a more greenish color. Velthaus et al.'s theory is that silver which has a single positive charge added to the phosphor as a co-dopant averages out the cerium to result in a net average charge of 2+ to provide charge compensation. In this way, the emission spectrum is shifted toward blue from what it would otherwise have been without the silver. However, SrS:Ag is not recognized by those designing phosphors suitable for alternating current thin film electroluminescent devices as being an efficient phosphors since its cathodoluminescent efficiency is poor when compared to that of SrS:Cu, e.g., 1% for SrS:Ag versus 10-15% for SrS:Cu. In other words, the silver acts to fill in the holes but is not considered a light-emitting dopant.
Thus, to date producers of thin film electroluminescent devices have yet to produce a blue emitting phosphor having sufficient luminance for use in a full color ACTFEL device.